Shi'ar Empire
The Shi'ar Empire is a vast group of species led by the Aerie and its current Emperor: Gladiator. The Shi'ar is one of the most important alien species in the Marvel Universe and is currently the most powerful after the demise of the Skrulls and the Kree.
The Shi'ar put Reed Richards on trial for the crimes of genocide. He was guilty of reviving Galactus after he was defeated on Earth. Shortly after his revival, Galactus proceeded to consume the Skrull throneworld, resulting in the death of billions. Uatu the Watcher, acting as his lawyer and with the help of Odin and Galactus himself, convinced the gathered tribunal that Galactus is a necessary force of betterment of the universe, not a villain. This was done by summoning Eternity. The truth as shown by Eternity however, is so grand and overpowering that none of the tribunal's members can remember it fully, although the comprehension stays.
Phoenix Saga
D'Ken Neramani, a corrupt Shi'ar ruler, attempted to use the powerful M'Kraan Crystal to take over the universe. His younger sister Lilandra and her new allies, the X-Men, foiled his plans. He was rendered comatose by the crystal, and Lilandra then took over as the Majestrix of the Shi'ar Empire. The X-Men, as well as most of Earth's other superheroes, have had cordial if not friendly relations with the Shi'ar Empire ever since.
Deathbird
In the 1980s, Lilandra and D'Ken's unstable exiled elder sibling, Deathbird, made several attempts to overthrow her sister from power. Deathbird even resorted to attacking Lilandra's Earth-based allies in order to achieve her goals. She is also responsible for initially directing the alien parasites known as the Brood towards the Earth and its heroes. Deathbird was eventually deposed with the assistance of the X-Men. In the 1992 crossover Operation: Galactic Storm, the Shi'ar annexed the Kree Empire at the end of the Kree-Shi'ar War and Deathbird was placed into a prominent position as viceroy of Hala, the Kree homeworld. However, Deathbird did not last long in this position and the current status of the Kree territories is unclear. The Shi'ar have had recent contact with the Spartoi, described as an equally advanced distant race, noted for Star-Lord's citizenship.
Cassandra Nova
Professor Xavier's evil twin sister, Cassandra Nova, single-handedly destroys a good portion of the Shi'ar Empire. Inhabiting the body of her brother, Nova asserts control of Empress Lilandra and causes a Shi'ar civil war. Jean Grey is instrumental in ending this threat.
Phoenix Endsong & End of Greys
Even though Jean Grey did the Shi'ar a big favor by eliminating the threat of Cassandra Nova, the Shi'ar still want her dead. In the Phoenix Endsong series, a group of Shi'ar tried to permanently kill the Phoenix Force and Jean Grey. Jean, however, escaped their suicide bomb attack and returned to the White Hot Room to restore herself. In the recent "End of Greys" story arc, the Shi'ar wanted to wipe out the Grey genome and Quentin Quire with the purpose of eliminating the possibility of a new Omega-level mutant becoming a host for the Phoenix Force. The Shi'ar Death Commandos murdered Jean Grey's father, niece, nephew, and other relatives in an alien invasion on Earth, thus inciting the wrath of Jean's daughter Rachel Summers, who has vowed vengeance on the entire Shi'ar Empire. Recent events seem to indicate the Shi'ar Council was responsible for this, and that Lilandra is unaware of what has been done in her name.
Captain Universe: Universal Heroes
After learning that The Uni-Power was malfunctioning Empress Lilandra and the Shi'ar Council dispatched Gladiator to Earth under the belief that it had become a threat to all civilized worlds and needed to be contained. Once he arrived on Earth, Gladiator came into conflict with The Fantastic Four and was forced to face Captain Universe (who had merged with The Invisible Woman) in combat, which nearly cost Gladiator his life. The Uni-Power then merged with the badly injured Gladiator and showed him that it did not pose a threat to anyone. After saving Los Angeles from a tsunami, Gladiator extended to The Uni-Power, an invitation to come back with him to The Shi'ar Throneworld as a guest while their scientists tried to solve The Uni-Power's dilemma.
Unfortunately while Gladiator and The Uni-Power were en route to the Empire both of them were captured by an energy-consuming reptilian space vampire who called himself Krosakis, who not only stripped Gladiator of all his power but also forced The Uni-Power into merging with him, thus turning Krosakis into Captain Universe. Luckily The Silver Surfer was able to sense The Uni-Power's plight and defeated Krosakis by overloading him with The Power Cosmic. The Surfer then permanently infused a portion of The Power Cosmic into The Uni-Power and repaired it, considering all they had been through over the years Gladiator believed The Surfer when he said The Uni-Power had been cured and allowed it to return to Earth while he took Krosakis back to The Shi'ar Throneworld so he could be placed on trial for his crimes.
Annihilation
As the war against Annihilus' invasion forces tore countless neighboring Star Empires asunder Peter Quill a.k.a. Star-Lord contacted The Shi'ar Empire on the behalf of Richard Rider a.k.a. Nova and his United Front hoping that The Shi'ar would help them bring Annihilus down once and for all. However The Shi'ar decided that it would be best, for them anyways, if they kept their position as a neutral party for the time being and would wait to see how the War played out.
Fall of the Shi'ar
A few years after the events of Phoenix: Endsong and Captain Universe: Universal Heroes; The Shi'ar were brutally assaulted by an X-Men villain called Vulcan. During his tenure as majestor of the empire, D'Ken killed Vulcan's mother Katherine Summers (also mother to long time X-Men Scott and Alex Summers) and made him a slave for most of his adolescent life. Bent on revenge against D'Ken, Vulcan attacks the Empire. Not only that, but within the empire, there is a coup to dethrone Lilandra and return D'Ken to power, with the aid of Deathbird. The X-Men once again team up with their space allies, the Starjammers, to stop both Vulcan and the plot to return rulership of the empire to D'Ken. In the end, Vulcan kills his father, Corsair, and D'Ken, and assumes the throne of the Shi'ar Empire for himself, with Deathbird as his queen; Lilandra and the Starjammers now lead a resistance against Vulcan's rule.
Emperor Vulcan